The present invention relates in general to methods making it possible to deposit, on a support formed of a first material, a coating layer comprising a second material or multiple materials with properties differing from those of the support material.
More precisely, the invention relates to a method for coating a support, comprising at least a deposition phase of coherent material.
“Coherent material” is understood to be a material having a coherent structure (crystallographic, for example): this definition thus excludes materials in the form of liquids or powders.
Even more precisely, the invention relates to a method making it possible to obtain a very high quality coating (particularly in terms of homogeneity, uniformity of surface state, and cohesion with the support), said method, compared with known methods, having significant advantages concerning rapidity and simplicity of implementation, and cost.
The coating of objects with a layer of material with specific properties (surface state, mechanical hardness, resistance to oxidation, compactness, adherence, seal, etc.) is extremely widespread in many fields (tooling, medical, industry, etc.), and many techniques are known for depositing a layer of coating material on an object.
A first type of classical technique, known by the acronym CVD (Chemical Vapour phase Deposition) consists in provoking a chemical reaction so as to cause a material to pass into gaseous phase, and then in condensing the obtained gaseous phase on a support in such a way that the material is deposited on the support.
The implementation of this type of technique necessitates controlling the temperature and pressure conditions in order to allow the changes of state of the material to be deposited on the support. Thus techniques for deposition under vacuum are known which allow improvement of the deposit and an increase in the quantities of deposited material.
But the quality of the coatings obtained by these known techniques is limited, because the molecules of the coating material are “deposited” on the support in the literal meaning of the term, the adherence of the coating to the support then being of medium quality, which may reveal unacceptable for certain applications.
Furthermore, these techniques do not make it possible to obtain coatings with an extremely smooth surface, which is required for many applications (for example for aesthetic considerations, or for using the advantages linked to a very low friction coefficient).
It is specified that the quality of the coating obtained depends on the one hand on the original state of the support surface, and on the other hand on the deposition method; in fact the different known methods can result in very different coating qualities.
To improve the performance of this first known technique type, it is also well known to create a low temperature plasma (with ions of energy lower than 1 eV).
The migration of the coating material over the support is controlled by the temperature of the support (which can be raised by thermal irradiation) or by bombardment with the ions extracted from the deposition plasma and used to enhance the growth of the coating (ion assisted deposition) by applying a negative electric potential to the support, for example from 50 to 200 eV.
To improve the coating, it is also known to raise the temperature of the support in such a way as to obtain better contact adhesion, or to use a supplementary ion beam of a few tens of keV accompanying the deposit. These methods are supplementary methods encouraging migration of the coating material over the support.
To implement these techniques, the plasma energy determines the surface temperature of the support, which is an important factor since this temperature must be sufficiently high to enable the atoms and molecules of the coating material to be mobile and to integrate correctly with the support so as to obtain good cohesion between the support and the coating and to ensure good homogeneity of the coating.
But it is also necessary to avoid a too important rise in the temperature of the support to cause a deterioration of the support itself.
In order to reconcile these two opposing restrictions, the coating must be made very slowly, in such a way as to allow the support to diffuse outwards part of the heat received. But this coating speed limitation constitutes a significant disadvantage for industrial application.
And in any case, the very fact of exposing the support to high temperatures makes this technique fairly difficult to control, because it requires special confinement means and procedures. Furthermore, it limits the range of materials from which the support can be made.
It is also known, in combination with the techniques mentioned above, to use a laser beam to enhance the deposition of molecules of coating material on the support (the laser having to be a high power laser in the case of utilisation with a plasma).
But this improvement has only very little effect on the disadvantages described above, since the deposition speed of the coating material remains low and the coating quality limited.
Thus, different techniques have been developed to improve the quality of the coating obtained. In particular are known:                a first type of technique using medium energy ion radiation (of the order of 20 to 30 keV) in combination with a plasma. This technique uses the energy of the ion radiation to stimulate the condensation of the coating material on the support, while respecting the crystallographic structure specific to the coating material, and enables a homogeneous spread of the coating material over the support.                    but the coating speed remains low in the case of this first technique type.                        a second type of technique using ultra-short bombardment pulses on the support by high energy density ion radiation (greater than 300 keV and associated with a fluence of the order of 1 J/cm2), which produces a higher quality coating (very homogeneous coating, able to be as smooth as glass), following the fusion of the superficial layer which mixes with the coating material during each ultra-short pulse, before solidifying by ultra-rapid cooling (a phenomenon known as “quenching”).                    However, it is difficult to envisage operating this second type of technique on an industrial scale, since it is slow, as like the techniques described above (the proportion of energetic ions bombarding the support during each pulse being only of the order of one per million particles of the unitary surface of the irradiated solid). Furthermore, the cost associated with producing very high-energy pulses at sufficiently high frequencies for making coatings of a significant thickness, is prohibitive.                        
Document U.S. Pat. No. 6,086,726 provides information about a method combining bombardment by an ion beam with one step of coating a support.
The method disclosed in this document thus comprises a single coating step, after which the coated support is bombarded with high-energy ions.
It is to be noted that in this method the ion bombardment can produce effects at very variable depths in the thickness of the coating layer and/or of the support.
This document indicates, in particular, that:                in certain implementation modes, said depth is of the order of only a tenth of the thickness of the coating layer, indicating that only part of this coating layer is affected by bombardment,        whereas in other implementation modes, said depth can be greater than the thickness of the coating layer. With regard to this, particular reference should be made in this document to column 4 lines 47 and following.        
Thus it appears that the method disclosed in this document does not include precise control of the ion bombardment conditions (and in particular the energy level of the bombarded ions), for obtaining a precise and precisely localised effect (for example between the coating layer and the coated support).
Moreover, it should be noted that this method comprises only a single coating step, and does not envisage any succession of several coating steps.
Also known, from document WO 99/65 038, is a method including the repetition of an operation consisting in successively adding powder on a support, then making at least one component of this powder melt through exposure to an ion beam, to create an alloy.
It will be noted that this method does not in any way involve a coating operation, since the deposited powder is not assimilated with a coherent coating of given structure such as that described below and concerned by the present invention.
In particular, in the method of document WO 99/65 038, the effect of ion bombardment is to give the layer of powder a structural coherence that is totally absent before this bombardment.
It is to be recalled that the invention lies within the framework of the improvement of coherent coatings.
And, on the contrary, the method of document WO 99/65 038 concerns a very special case of utilisation of a powder (non-coherent), and the creation of a coherent structure through exposure to an ion beam.
Finally, from document U.S. Pat. No. 4,759,948, a method is known which is intended to make a surface to be coated uniform, said method involving exposure of the support surface to be coated to an ion beam.
This method thus only provides information about a preparatory step before a coating.
And it should be noted that the energy of the ion beam is consequently not adapted to causing fusion of the surface.
Thus it can be seen that as a whole the known methods have restrictions, or are not adapted to the aim of the invention.